As an example of a conventional nonvolatile semiconductor storage device, a flash memory will be described below. An example of an outline of the configuration of a conventionally common flash memory is shown in FIG. 5.
In this example, each memory array cell 51 is composed of a large number of flash memory elements, and feeds the outputs of 1,024 bit lines to a multiplexer 52. Each multiplexer 52 is, on its output side, connected to sense amplifiers 53 that are provided one for every 128 bit lines. That is, each multiplexer 52 is, on its output side, connected to eight sense amplifiers 53. To the inverting input terminal of each sense amplifier 53 is connected a reference cell 54 that outputs a reference current. Here, each sense amplifier 53 is built as a current-to-voltage conversion amplifier.
This conventional flash memory includes a plurality of circuit blocks each including a memory array cell 51, a multiplexer 52, sense amplifiers 53, and reference cells 54 (FIG. 5 shows only two such circuit blocks).
In a read operation from the conventional flash memory shown in FIG. 5, first, from among the plurality of memory array cells, one with which to perform the read operation is selected, and the selected memory array cell then transfers the data stored therein to the multiplexer via 1,024 bit lines. The multiplexer 52 then outputs one chunk after another of the data fed thereto from the memory array cell to the individual sense amplifiers 53. Each sense amplifier 53 then compares the reference current fed from the reference cell 54 with the current fed from the multiplexer, and outputs a voltage that is commensurate with the difference between those currents.
In the flash memory shown in FIG. 5, eight sense amplifier are provided for each memory array cell, and a read operation is performed as described above. Consequently, a single read operation accomplishes the reading of as little as eight-bit data.
By providing more sense amplifiers for each memory array cell, it is possible to increase the number of data bits that can be read in a single read operation. This, however, is undesirable from the viewpoint of compactness because the sense amplifiers, which are of the current-to-voltage conversion type, require a large circuit area. Hence, it is unpractical to provide more than 16 sense amplifiers for each memory array cell. This limits the number of data bits that can be read from a conventional flash memory in a single read operation to eight to sixteen. This is the reason that read operation tends to be slow with conventional flash memories.